Thermal energy in the ground is commonly used for heat exchange processes. Below a certain depth (e.g., 30 feet), the underground temperature consistently approximates the average annual temperature. The underground temperature generally does not vary based on the season. Thus, in warmer seasons, the underground region can serve as a heat sink, while in colder seasons, the underground region can serve as a heat source. Fluid (e.g., water) can be directed underground to either shed heat or extract heat. The fluid can then be directed back above ground for various heating/cooling purposes. Such ground heat exchange systems are becoming more common and cost-effective. Traditionally, up-front costs associated with digging wells and installing equipment have served as significant impediments to the large-scale adoption of geothermal heat exchange systems. Once installed, however, such systems can considerably reduce ongoing energy costs, but the large up-front costs can result in long payback periods.
Ground heat exchange systems sometimes include a borehole located in geologic units, a tube, a heat-exchange fluid that flows down through the annulus between the borehole and the tube and back up inside the tube. Such configurations are referred to as an open-loop ground heat exchange system as the fluid can mix with the surrounding aquifers as it travels down the annulus between the borehole and the inner tube. When a borehole encounters multiple aquifers in surrounding geologic units, the open-loop ground heat exchange system can allow the aquifers to interconnect. In many instances, regulations prohibit the interconnection of aquifers.
Alternate embodiments of ground heat exchangers may have a closed-loop configuration. In this configuration, no external fluid is introduced into the system. In such closed-loop configurations, filtration is not necessary because the fluid does not mix with the surrounding aquifers. In addition, in some embodiments, inhibitors and/or antifreeze may be added to the fluid to enhance heat transfer without such inhibitors and/or antifreeze mixing with the underground water.